MySQL

multithreaded, multi-user SQL database management system which has about 10 million installations. MySQL works on many different platforms including AIX, BSDI, FreeBSD, HP-UX, Linux, Mac OS X, NetBSD, Novell NetWare, OpenBSD, OS/2 Warp, QNX, SGI, IRIX, Solaris, SunOS, SCO OpenServer, SCO UnixWare, TRu64, Windows 95, Windows 98, Windows ME, Windows NT, Windows 2000, Windows Xp, and Windows Vista